1. Field of the Invention
The present invention is directed toward the field of discrete passive filters, and more particularly toward a tunable LC filter bank.
2. Art Background
Typically, receivers employ filters to condition both input signals and internally generated reference signals. For example, bandpass, notch, and low pass are types of filters employed in receivers. The frequency response of a filter refers to the characteristics of the filter that condition the signal input to the filter. For example, a bandpass filter may attenuate an input signal across a pre-determined band of frequencies above and below a center frequency of the filter. Filters are designed to exhibit frequency responses based on one or more circuit parameters.
Some receivers are designed to process input signals with a range of input carrier frequencies (e.g., wide band receivers). For example, television receivers must be capable of processing input television signals with carrier frequencies ranging from 55 MHz to 880 MHz. One circuit parameter used to define the frequency response of a filter is the carrier frequency of an input signal. Thus, such wide band receivers require filters to generate multiple frequency responses to accommodate multiple input carrier frequencies. To accomplish this, some receivers employ tunable filters to process a wide band of input frequencies.
One type of tunable filter is a varactor type tuner. A popular application for the varactor is in electronic tuning circuits, such as television tuners. A direct current (xe2x80x9cDCxe2x80x9d) control voltage varies the capacitance of the varactor, re-tuning a resonant circuit (i.e., filter). Specifically, a varactor diode uses a pn junction in reverse bias such that the capacitance of the diode varies with the reverse voltage. However, the relationship between the control voltage and the capacitance in a varactor tuner is not linear. Thus, the capacitance value is based on the signal level. This non-linearity produces distortion in the output of the filter (e.g., a third order product).
Other receivers, such as television receivers, may employ active filters. The use of a continuous or active filter requires a power supply voltage (e.g., Vcc). The power supply voltage exhibits a ripple due to noise on the voltage supply line. This ripple voltage, in turn, causes unacceptable frequency response characteristics on the output of the continuous amplifier. Accordingly, it is desirable to use discrete or passive filters in the receiver to isolate the signal from ripple voltage, thereby improving signal quality.
A receiver, such as a television receiver, tunes an inductive (xe2x80x9cLxe2x80x9d) and capacitive (xe2x80x9cCxe2x80x9d) filter based on a desired frequency. The LC filter includes a plurality of inductors, configured in at least one inductive (xe2x80x9cLxe2x80x9d) bank, and a plurality of capacitors configured in at least one capacitive (xe2x80x9cCxe2x80x9d) bank. The inductors are selectively enabled for the LC filter by an N code, and the capacitors are selectively enabled for the LC filter by an M code.
In order to tune the receiver for a channel frequency in the VHF spectrum, the receiver determines a first offset between an initial resonant frequency of the LC filter and the channel frequency (i.e., the desired frequency for which the LC filter is tuned). For course tuning, the N code is determined based on an initial value for the M code, the desired frequency, and the first offset. At least one inductor in the L bank is selected based on the N code. Then, a second offset between a resonant frequency of the LC filter and the desired frequency is determined. For fine-tuning, the M code is determined based on the value for the N code, the desired frequency, and the second offset. At least one capacitor in the C bank is selected using the M code.
In order to tune the receiver for a channel frequency in the UHF spectrum, the receiver also determines a first offset between an initial resonant frequency of the LC filter and the desired frequency. For course tuning, the M code is determined based on an initial value for the N code, the desired frequency, and the first offset. At least one capacitor in the C bank is selected from the M code. A second offset between a resonant frequency of the LC filter and a desired frequency is determined. To fine-tune the LC filter, the N code is determined based on the value for the M code, the desired frequency, and the second offset. At least one inductor in the L bank is selected from the N code.